idq_aggregator.py: changed hdf saving scheme for datasets (saves multiple datasets per file keyed by channel and rate), hdf file saving for non-aggregates 2x faster, fixed bug with aggregation that caused missing triggers; gstlal_idq_trigger_gen: major refactoring in buf_handler and hdf saving functions

idq_aggregator.py: changed indexing scheme from MultiIndex to DatetimeIndex, multilevel aggregation fixed (except for possibly some edge cases)

- utils --> ligolw_utils

- introduce internal cacheentry class to hold entry information
- remove limit on cache size
- when initializing, move loop over instruments into .add_to_cache()
- add min_instruments parameter to the cache key
- in P_instruments_given_signal(), handle off instruments better
- record P(instruments | signal) in the SNRPDF cache
- enables generalization of ranking statistic away from fixed P(instruments | signal) marginalized over horizon distance history

- replace some generator expressions with lists that become generator
  expressions again in Python 3

- seems to be the source of the plague of lock-ups we've been suffering from

- *** REVERT AFTER O2 ***

tree.py: put prefactor to 1 and pad the equal mass line as well as remove mass scaling of deltas and increase metric tolerance

gstlal_idq_trigger_gen: added option to turn on hdf file writing with dataframes, changed string concatenation scheme for writing trigger ascii files to disk when using dataframes